1. Technical Field
The present invention relates to a piezoelectric film with little oxygen defect and having excellent piezoelectric characteristics and a method for manufacturing the same, a piezoelectric film element and a method for manufacturing the same, and a piezoelectric film device.
2. Description of the Related Art
The piezoelectric film is processed into various piezoelectric film elements for different kinds of purposes, and is widely utilized as functional electronic components such as particularly an actuator that causes deformation to occur by applied voltage, and a sensor that generates a voltage from the deformation of the element reversely. A lead-based dielectric having excellent piezoelectric characteristics and particularly Pb(Zr1-xTix)O3-based perovskite ferroelectric called PZT is widely used as a piezoelectric utilized for the usage as the actuator and the sensor. Such a perovskite ferroelectric is usually formed by sintering an oxide composed of individual elements.
Further, in recent years, a development of a piezoelectric not containing lead is desired in consideration of an environment, and a development of lithium potassium sodium niobate (general formula: (NaxKyLiz)NbO3(0<x<1, 0<y<1, 0<z<1, x+y+z=1), etc., is progressed. The lithium potassium sodium niobate has the piezoelectric characteristics corresponding to PZT, and therefore is expected as a strong candidate of a lead-free piezoelectric material.
Meanwhile, as miniaturization and higher performance of each kind of electronic component are achieved, miniaturization and higher performance of the piezoelectric element are also strongly desired. However, in a piezoelectric material fabricated by a manufacturing method focusing on a sintering method, which is a conventional method, when a thickness of the material becomes particularly 10 μm or less, a size of the material is close to a size of a crystal grain that constitutes the material, and an influence of the size can't be ignored, thus involving a problem that variation of the characteristics or deterioration becomes remarkable. In order to prevent such a problem, a method for forming a piezoelectric utilizing a thin film technique instead of the sintering method, has been studied in recent years.
In recent years, a PZT thin film formed on a silicon substrate by a RF sputtering method, is put to practical use as an actuator for a head of a fast high-resolution inkjet printer (for example, see patent document 1).
Further, a piezoelectric film with a part of Pb replaced by Bi (bismuth) is put to practical use as a small and inexpensive gyro sensor or angular velocity sensor (for example see patent document 2). According to the patent document 2, for example, the piezoelectric film formed by the sputtering method is capable of obtaining a high piezoelectric constant by replacing part of Pb by Bi, even in a case of a composition having excess or deficiency of oxygen.
Further, patent document 3 proposes a piezoelectric film having high orientation in a specific plane direction, wherein a piezoelectric thin film element of lithium potassium sodium niobate not using lead is expressed by a general formula (NaxKyLiz)NbO3. According to the patent document 3, for example, the piezoelectric thin film formed by the sputtering method is lead-free, having excellent piezoelectric characteristics.    Patent document 1:    Japanese Patent Laid Open Publication No. 1998-286953    Patent document 2:    Japanese Patent Laid Open Publication No. 2005-333088    Patent document 3:    Japanese Patent Laid Open Publication No. 2007-19302
Incidentally, the sputtering method is frequently used, which is a proven technique as a mass production in forming the piezoelectric film. As shown in FIG. 11, the sputtering method is a method for forming a thin film 24 by plasma-ionizing argon gas as a kind of an inert gas in a vacuum environment of a sputtering device 20, then colliding the argon ion against a target 21, being a sintered body constituted of the same element composition as the composition of the piezoelectric film, so that sputtering particles 22 that fly out from the target 21 by collision of the argon ion, are adhered to a substrate 23 which is opposed to the target 21.
In this sputtering method, film formation is carried out under a high vacuum state in principle, and therefore oxygen is insufficient in the formed piezoelectric film in many cases. Namely, in the piezoelectric film formed by the sputtering method, oxygen is stoichiometrically reduced compared with a raw material target, thus generating a compositional deviation of the oxygen.
Further, when the piezoelectric film device is fabricated, there is a case that the piezoelectric film is subjected to heat treatment in a reduction atmosphere, or a case that a molecule constituted of water, etc., having hydrogen and a hydroxyl group is used as a raw material when an insulating layer and a protective layer are formed in a peripheral part of the piezoelectric film. In this cases, the piezoelectric film allows a reductive reaction to be promoted by reaction between hydrogen, being an atmosphere gas, and oxygen in the piezoelectric film, on an interface between the piezoelectric film and an electrode made of Pt having catalytic activity. As a result, the oxygen in the piezoelectric film is desorbed, thus generating the compositional deviation of the oxygen.
The compositional deviation of oxygen is caused by an oxygen defect generated in a local structure around each atom constituting the piezoelectric film. The oxygen defect is a lattice defect generated by fluctuation of a bonding state of an atomic level in the piezoelectric film, which is caused by the reductive reaction. The piezoelectric film having the oxygen defect has a low piezoelectric characteristic, because it is difficult to form a perovskite structure.
From this point, it can be considered that in a case of the PZT-based piezoelectric film of the patent document 2, high piezoelectric characteristic can be obtained even if having excess/deficiency of oxygen (compositional deviation of oxygen), but the PZT-based piezoelectric film includes lead, and it is difficult to improve the piezoelectric characteristics by suppressing the oxygen defect. Further, in a case of the piezoelectric film of the patent document 3, it is also difficult to improve the piezoelectric characteristics by suppressing the oxygen defect, although the piezoelectric film does not include lead.